CA1073655A - Method and composition for acidizing subterranean formations - Google Patents

Abstract

METHOD AND COMPOSITION FOR ACIDIZING SUBTERRANEAN FORMATIONS

Abstract of the Disclosure Gelled acidic compositions suitable for either matrix acidizing or fracture-acidizing of subterranean formations, and methods of using com-positions in acidizing operations, are provided. Said compositions comprise water, a water-dispersible polymer of acrylamide, an acid, and an aldehyde capable of causing gelation of an aqueous dispersion of said polymer, said acid, and said aldehyde.

Description

``- 10736~5 24,837 METHOD AND COMPOSITION FOR ACIDIZING SUBTERR~NEAN ~ORMATIONS
This invention relates to acid treating or acidizing of subterran-ean formations.
Acid treating or acidizing of porous subterranean formations pene-trated by a well bore has been widely employed for increasing the production of fluids, e.g., crude oil, natural gas, etc., from said formations. The usual technique of acidizing a formation comprises introducing a non-oxidi-zing acid into the well under sufficient pressure to force the acid out into the formation where it reacts with the acid-soluble components of the forma-tion. The technique is not limited to formations of high acid solubility such as limestone, dolomite, etc. The technique is also applicable to other types of formations such as a sandstone containing streaks or striations of acid soluble components such as the various carbonates.
During the acid treating operation, passageways for fluid flow are created in the formation, or existing passageways therein are enlarged, thus stimulating the production of fluids from the formation. This action of the acld on the formation is often called etching. Acid treating or acidizing operations wherein the acid is injected into the formation at a pressure or rate insufficient to create cracks or fractures in the formation is uaually referred to as matrix acidizing.
Hydraulic fracturing is also commonly employed to increase the production of fluids from subterranean formations. ~ydraulic fracturing comprises the injection of a suitable fracturing fluid down a well penetrat-ing a formation and into said formation under sufficient pressure to over-come the pressure exerted by the overburden. This results in creating a crack or fracture in the formation to provide a passageway which facilitates flow of fluids through the formation and into the well. Combination frac- -ture-acidizing processes are well known in the art.
Thus, it is within the scope of the present invention to inject the gelled acidic compositions of the invention into the formation under in-sufficient pressure to cause fracturing of the formation and carry out a -1- ~

" 1~73655 m~trix acidizing operation, or inject said gelled acidic composition at suf-ficient rates and pressure to cause fracturing and carry out a combination fracture-acidizing operation.
One of the problems co~monly encountered in acidizing operations is insufficient penetration of the formation by the acid. It is desirable that good pene~ration be obtained in order to realize maximum benefits from the operation. Too often the acid is essentially completely spent in the area immediately adjacent and surrounding ~he well bore. m e severity of the problem increases as the well temperature increases because acid reac-tivity with the formation increases with increasing temperatures, as in deeper wells.
Poor penetration can also be caused, and/or aggravated, by fluid loss to the more porous zones of the formation where low permeability is not a problem. Poor penetration can also be caused, and/or aggravated, by leak-off at the fracture faces in combination fracturing-acidizing operations.
Either said fluid loss or said leak-off can frequently worsen the situation by leaving the tight (low permeability) zones of the formation unchanged and merely opening up the already high permeability zones.
One solution which has been proposed for the above discussed prob-lem is to incorporate various polymeric thickening or viscosifying agents into the acid solution. Said agents serve to thicken the acid solution and thus increase the viscosity thereof. It has been reported that so thickened acid solutlons have reduced fluid loss properties. For example, see U.S.
Patent 3,415,319 issued DeceTnber 10, 1968 in the name of B. L. Gibson; and U.S. Patent 3,434,971 issued March 25, 1969 in the name of B. L. Atkins. It has also been reported that the reaction rate of said so-thickened acid so-lutions with the acid-soluble portions of the formation is lessened or re-tarded. See, for example, U.S. Patent 3,749,169 issued July 31, 1973 in the name of J. F. Tate; U.S. Pat~nt 3,236,305 issued February 22, 1966 in the name of C. F. Parks; and U.S. Patent 3,252,904 issued May 2~, 1966 in the name of N. F. Carpenter.

- 2 -~73~55 Higher viscosities are also advantageous in combination fractur-ing-acidizing operations in that the more viscous acidic solutions produce wider and longer fractures. More viseous acid solutions are also more ef-fective in carrying propping agents into the formation when propping agents are used.
Another problem encountered in aeidizing operations, partiGularly when employing acidizing compositions having thickening or viscosifying agents incorporated therein, is stability to heat. By stability to heat, it is meant the retention of the increased or greater viscosity properties under the conditions of use. Such compositions to be satisfactory should be suf-ficiently stable to resist degeneration by the heat of the formation for a period of time sufficient to accomplish the intended purpose, e.g., good penetration and significant etching of the formation. The degree of stabil-ity required in any particular operation will vary with such operating var-iables as the type of formation being treated, the temperature of the forma-tion, the well depth (time to pump the acidic composition down the well and into the formation), the acid concentration in the compositlon, etc. For example, acidizing of a tight low permeability formation will proceed more slowly than a more permeable formation, other factors being the same, be-cause a l~nger time will be required to obtain a significant amount of etch-ing and the composition must remain in place and effective for a longer per-iod of time. Also, more time will be required to pump the acidic composition into place in the formation.
The temperature of the formation usually has a pronounced effect on the stability of the acidizing compositions and, generally speaking, is one of the most important operating variables when considering stability.
Increased formation temperatures usually have at least two undesirable ef-fects. One such effect is degeneration of the composition, e.g., decrease in viscosity. Another such effect is increased rate of reaction of the acid with the formation. Thus, some compositions which would be satisfactory in 1~73~S~
a low temperature formation such as in the ~lugoton field in the Anadarko basin might not be satisfac~ory in formations encountered in deeper wells as in some ~est Texas -fields.
In ordinary acidizing operations using unthickened acids there is usually no problem in removing the spent acid because it is essentially water. However, a problem ~hich is sometimes encountered when using thick-ened compositions in treating formations is the ease of removal of the treat-ing composition after the operation is comple~ed. Some thickened or highly viscous solutions are difficult to remove from the pores of the formation or the fracture after the operation is complete. Sometimes a clogging residue can be left in the pores of the formation, or in the fracture. This can in-hibit the production of fluids from the formation and can requixe costly cleanup operations. It would be desirable to have gelled acidic composi-~ions which break down to a lesser viscosity withLn a short time after the operation is completed.
The present invention provides a solu~ion for, or at least mlti-gates, the above discussed problems. The present invention provides im-proved methods for acidizing, or fracture-acidizing, subterranean formations;
and new gelled acidic compositions for use in said methods.
Thus, in accordance with one broad aspect of the concept of the invention, there is provided a method for acid treating a porous subterran-ean formation susceptible of attack by an acid and penetrated by a well bore, which method comprises: injecting into said formation via said well bore a gelled acidic composition comprising water; a water thickening amount oE a water-disperslble polymer selected from the group consisting of polyacryl-amides and polymethacrylamides; partially hydrolyzed polyacrylamides and polymethacrylamides wherein a portion of the carboxamide groups are initially hydrolyzed to carboxyl groups; crosslinked polyacrylamides and crosslinked polymethacrylamides; partially hydrolyzed crosslinked polyacrylamides and partially hydrolyzed crosslinked polymethacrylamides wherein a portion of the carboxamide groups are initially hydrolyzed to carboxyl groups; copoly-mers of acrylamide or me~hacrylamide with another ethylenically unsaturated : . ' ' ', -.' ~ ' '' ' ' . :' ' 1~73655 monomer copolymerizable therewi~h, sufficient acrylamide or.methacrylamide being present in the monomer mixture to impart said water-dispersible prop-erties to the resulting copolymer when it is mixed with water; and mixtures thereof;
an amount of an acid which is capable of, and sufficient for, re-acting with a significant amount of the acid-soluble components of said formation; .
a small but effective amount of a mixture of at least two water-dispersible aldehydes which is sufficient to cause gelation of an aqueous dispersion of said polymer, said acid, and said aldehydes;
said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion there-of to permit said gelation and thus form a said composition having sufficient stability to degeneration by the heat of said formation to permit good pen-etration of said composition into said formation; and maintaining said composition in said formation in contact there-with for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.
Further, in accordance with another broad aspect of the concept of the invention there is provided a gelled acidic composition, suitable for matrix acidizing or fracture-acidizing of a subterranean formation, compris-ing: water; a water-thickening amount of a water-dispersible polymer selected Erom the group consisting of polyacrylamides and polymethacrylamides wherein up to about.45 percent of the carboxamide groups can be initially hydrolyzed to carboxyl groups; crosslinked polyacrylamides and crosslinked polymeth-acrylamides wherein up to about 45 percent of the carboxamide groups can be initially hydrolyzed to carboxyl groups; copolymers of acrylamide or meth-acrylamide with another ethylenically unsaturated monomer copolymerizable therewith, sufficient acrylamide or methacrylamide being present in the mon-omer mixture to impart said water-dispersible properties to the resulting ~L~7~65~

copolymer when it is mixed with ~ater; and mixtures thereof; an amount of a non-oxidizing acid which is capable of, and sufficient for, reacting with a significant amount of the acid-soluble components of said formation; a small but effective amount of a mixture of at least two water-dispersible alde-hydes which is sufficient to cause gelation of an aqueous dispersion of said polymer, said acid, and said aldehydes; said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereo~ to permit said gelation and thus form a said composition having sufficient stability -to degeneration by the heat of said formation to permit g~od penetration of said composition into said formation and the maintenance of said composition in said formation in contact therewith for a period of time sufficient for the acid in said com-position to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.
Still further, in accordance with other broad aspects of the in-vention, there are provided methods ~or preparing said gelled acidic compo-sitions.
In some embodiments of the invention only one aldehyde can be used, if desired, instead of a mixture of at least two aldehydes.
As noted above, the gelled acidic compositions of the invention must be suitable for matrix acidizing or fracture-acidizing of subterranean formations. In order to satisfy this require~ent, the polymer, the acid, and the aldehyde(s), in the amounts used, must be sufficiently compatible with each other, in an aqueous dispersion thereof, to permit the gelation of said dispersion and thus form a said composition having sufficient sta-bility to degeneration ~y the heat of the formation to permit good penetra-tion of said composition into the formation. Furthermore, once said pene-tratlon has been attained, the said stability must be sufficient to permit the maintaining of said composition in contact with the formation for a per-iod of time sufficient for the acid in the composition to significantly re-act with the acid-soluble components of the formation and stimulate the ~73655 production of flulds therefrom, e.g., by creating new passageways or enlarg-ing existing passageways through said for~ation.
Herein and in the claims, unless otherwise specified, the term "good penetration" means penetration of live or effective acid into the formation a sufficient distance to result in stimulating the production of fluids therefrom, e.g., by the creation of sufficient ne~ passageways, or sufficient enlargement of existing passageways, through said formation to significantly increase the production of fluids fr~m the for~nation. This can vary for different formations, well spacings, and what it is desired to accomplish in a given acidizing t~eatment. Those skilled in the art will usually know what will be "good penetration" for a given formation and a given type of treatment. However, generally speaking, for guidance purposes in the practice of the invention and not by way of limitation of the inven-tion, "good penetration" will usually be considered to be a distance of a few feet, e.g., up to 5 or more, in a small volume matrix acidizing opera-tion, and several hundred feet, e.g., up to 500 or more, in a large volume fracture-acidizing operation.
Herein and in the claims, unless otherwise specified, the term "polymer" is employed generically to include both homopol~ners and copoly-mers; and the term "water-dispersible pol~ners" is employed generically to include those polymers which are truly water-soluble and those polymers which are dispersible in water or other aqueous medium to form stable col-loidal suspensions which can be gelled as described herein. Also, the term "aqueous dispersion" is employed generically to include both true solutions and stable colloidal suspensions of the components of the compositions of the invention which can be gelled as described herein.
~ ny suitable polymer of acrylamide meeting the above stated com-patibility requirements can be used in the practice of the invention. Thus, under proper conditions of use, sueh polymers can include various polyaeryl amides and related polymers which are water-dispersible and which can be used in an aqueous medium, with the gelling agents described herein, to give -" ~11)73~jS~i;
an aqueous gel. These can include the various substantially linear homo-polymers and copolymers of acryla~ide and meth~crylamide. By substantially linear it is meant that the polymers are substantially free of crosslinking between the polymer chains. Said polymers can have up to about 45, prefer-ably up to about 40, percent of the carboxamide groups hydrolyzed to car-boxyl groups. Generally speaking, as the degree of hydrolysis increases, the polymers tend to become more difficu~lt to disperse in aqueous acidic media. Thus, one presently more preferred group of polymers includes those wherein not more than about 20 percent of the carboxamide groups are hy-drolyzed. As used herein and ln the claims, unless otherwise specified, the term "hydrolyzed" includes modified polymers wherein the carboxyl groups are in the acid form and also such polymers wherein the carboxyl groups are in the sal~ form, provided said salts are water-dispersible. Such salts in-clude the ammonium salts, the alkali metal salts, and others which are water-dispersible. Hydrolysis can be carried out in any suitable fashion, for ex-ample, by heating an aqueous solution of the polymer with a suitable amount of sodium hydroxide.
As used herein and in the claims, unless otherwise specified, the stated values for "degree o~ hydrolysis" or "percent hydrolyzed", and like terms, refer to initial values prior to use or test of the polymer. Unless otherwise stated, said values were obtained by the following analytical pro~
cedure. Place 200 ml of distilled water in a beaker provided with a magnetic stirrer. Weigh a 0.1 gram polymer sample accurately to i 0.1 mg. Start the stirrer and quantita~ively transfer the weighed sample into the water vortex.
Stir at a rapid rate overnight. Using a pH meter and a 1:1 diluted HCl, ad-~ust the pH of the sample solution to less than 3Ø Stir the solution for 30 minutes. Adjust the pH of the solution to exactly 3.3 by dropwise addi-tion of 0.1 N NaOH. Then slowly titrate with standard 0.1 N NaOH from pH

3.3 to pH 7Ø
V x ~ x Q.Q72 x 100 % Hydrolysis = W
where: V = ml of base used in titration; N = normality of base; W = grams of polymer sample, and 0.072 ~ milliequivalent weight of acrylic acid.

``` ~B73655 Substan~ially linear polyacrylamides can be prepared by methods known in the art. For exampla, the polymerization can be carried out in aqueous medium, in the presence of a small but effective amount of a water-soluble oxygen-containing catalyst, e.g., a thiosulfate or bisulfate of potassium or sodium or an organic hydroperoxide, at a temperature between about 30 and 80C. The resulting polymer is recovered from the aqueous med-ium, as by drum drying, and can be subsequently ground to the desired part-icle size. The particle size should be ~ine enough to facilitate dispersion of the polymer in water. A presently preferred particle size is such that about 90 weight percent will pass through a number l0 mesh sieve, and not more than about 10 weight percent will be retained on a 200 mesh sieve (U.S.
Bureau of Standards Sieve Series).
Under proper conditions of use, examples of copolymers which can be used in the practice of the invention can include the water-dispersible copolymers resulting from the polymerization of acrylamide or methacrylamide with an ethylenically unsaturated monomer copolymerizable therewith. It is desirable that sufficient acrylamide or methacrylamide be present in the monomers mixture to impart to the resulting copolymer the above-described water-dispersible properties. Any suitable ratio of monomers meeting this condition can be used. Under proper conditions of use, examples of suitable ethylenically unsaturated monomers can include acrylic acid, methacrylic acid, vinylsulfonic acid, vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, vinyl acetate, acrylonitrile, methyl acrylonitrile, vinyl alkyl ether, vinyl chloride, maleic anhydride, vinyl substituted cationic quaternary ammonium compounds, and the like. Various methods are known ln the art for preparing said copolymers. For example, see U.S. patents 2 9 625,529 issued January 13, 1953 in the name of R. M. Hedrick; 2,740,522 issued April 3, 1956 in the name of F. M. Aimone et al, 2,727,557 issued December 20, 1955 in the name of R. L. Fox; 2,831,841 issued April 22, 1958 in the name of G. D. Jones; and 2,909,508 issued October 20, 1959 in the name of G. D.
Jones. Said copolymers can be used in the hydrolyzed form, as discussed above for the homopolymers.

~L~B73~55 One presently preferred group of copolymers for use in the prac-tice of the invention are the'copolymers of acrylamide or methacrylamide with a monomer of the formula '~
O :
R - C - C - N - R' - SO3M

(A~
wherein: R is hydrogen or a lower alkyl radical containing from 1 to 6 car-bon atoms, said R preferably being hydrogen or a methyl radical; ~' is an alkylene radical containing from 1 to 24 carbon atoms or an arylene radical containing from 6 to 10 carbon atoms, said R' preferably being an alkylene radical containing from 2 to about 10 carbon atoms; and M is hydrogen, am-monium, or an alkali metal, said M preferably being hydrogen, æodium, or potassium; and wherein the number of repeating units from said formula (A) monomer is within the range of from 1 to 90, preferably 5 to 80, more pre-ferably 10 to 70, mol percent.
Monomer~ of the above formula (A) and methods for their prepara-tion are known in the art. For example, see U.S. Patent 3,507~707, issued -April 14, 1970 in the name of L. E. Miller et al; and U.S. Patent 3,768,565, issued October 30, 1973 in the name of L. J. Persinski et al. In the above formula (A), when'R is hydrogen, R' is C~13 and M is hydrogen, said monomer is the well known AMPS (trademark) monomer, '' 2-acrylamido-2-methylpropanesulfonie acid, which is available commercially from The'Lubrizol Corporation, Cleveland, Ohio. The al~ali metal salts of said monomer, e.g., soditlm 2-acrylamido-2-methylpropane sulfonate, are also readily available.
Copolymers of acrylamide with said AMPS monomer, and/or its sod-ium salt,are known. For example, see the'above-mentioned Persinski patent.

: -- , ,, - . ~ , : . .
.
~ . . ' " . ~ . ' ' ... ., . . ~ .. . . . . .

~0736~5 A number of said copolymers are also available from Hercules Incorporated, Wilmington, Delaware. For example, ~lercules SPX-5024 (proprietary designa-tion), a 90:10 acrylamide/AMPS sodium salt copolymer; Hercules SPX-5022 (proprietary designation), an 80:20 acrylamide/AMPS sodium salt copolymer;
Hercules SPX-5023 (proprietary designation), a 50:50 acrylamide/AMPS sodium salt copolymer; and Hercules SPX 5025 (proprietary designation), a 30:70 acrylamide/AMPS sodium salt copolymer The above type of copolymers wherein the number of units from said formula (A) monomer is within the range of from 10 to 70 mol percent, thus comprise one presently more preferred group of co-polymers for use in the practice of the invention. Said copolymers can berepresented by the formula __. -CH2---C~ -CH2---C~ ------------- ---O = C O = C CH3 NH~ 'x HN - C ~ CH2 ~ S03Na Y

(B) wherein x and y represent the 1 percent of said units as set forth above, it being understood that the various copolymers do not necessarily consist of alternating units as depicted above in (B). It is also within the scope of the invention for the acrylamide units in the above formula (B) to be methacrylamide units, and for a portion of the -Nnl2 groups in said units to be hydrolyzed.

Thus, it is also within the scope of the invention for the acryl-amide units in the above formula (B) to be derived from either acrylamide or methacrylamide wherein the -NH2 group can be NH2 or -OM as defined below.
m us, copolymers of said derivatives with the above monomer (A) can be repre-sented by the formula : . . .: . . , , : , . . .,: . :

~3t;~5 . R" R , __ ---CH2 - C- -------- ---CH2 - C------------ --- -O=C O=C

_ _ x H~ - R' - SO3M Y
I II -10 .(B') .
wherein: R, R', and M are as defined above in formula (A); R" is hydrogen or a methyl radic~l; Z is either -NH2 or -OM in the above Type I monGmer units, with the proviso that the copolymer contains at least 10 mol percent of said Type I monomer units in which Z is -NH2; and x and y are the mol percent val-ues of the respective indi~idual monomer units I and II, with x being in the range of from 10 to 99, preferably 20 to 95, more preferably 30 to 90, and with y being in the range o~ from 1 to 90, preferably 5 to 80, more prefer- :
ably 10 to 70; and wlth it ~eing understood that the various copolymers do ~ .
not necessarily consist of alternating monomer units as depicted in form~la (B'), e.g., the copolymers are random copolymers as represented by the broken lines connect-lng said monomer units. It is presently believed that in the above copolymers it is desirable that there be at least 10 mol percent of ;~
monomer units containing the -CONH2 group in order for gelation to take place in the presence of an aldehyde gelling agent in accordance with the inven-tion.
Another presently preerred group of copolymers for use in the practice of the invention are the copolymers of acrylamide or me~hacrylamide with a monomer of the formula O R" +
" .
R - C - C - O - R' - N - R" X

C~2 R"

(C) wherein: R is hydrogen or a lower alkyl radical containing rom 1 to 6 car-bon atoms, said R preferably being hydrogen or a methyl radical; R' ls an :. .,: . ..,: . , . ., : :, . : .
.... ; : ... : .: .: : ~ :

1~3655 alkylene radical containing from 1 to 24 carbon atoms or an arylene radical containing from 6 to 10 carbon atoms, said R' preferably being an alkylene radical containing from 2 to about 10 carbon atoms; each R" is an alkyl rad-ical containlng from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms;
X is any suitable anion such as methylsulfate, ethylsulfate, chloride, bro-mide, acetate, nitrate, and the like; and wherein the number of repeating units from said formula (C) monomer is within the range of from 1 to 90, pre-ferably 5 to 70, re preferably 10 to 60, mol percent.
Monomers of the above formula (C) and methods for their prepara-tion are known in the art. For example, see U.S. Patent 3,573,263, issued March 30, 1971 in the name of E. A. Gill. In the above formula (C), when R
is H, R' is -CH2-OEl2-, one R" is a methyl radical and the other two R" are each an ethyl radical, and X is a CH3S04 anion, the monomer is the commer-cially available material (acryloyloxyethyl)diethylmethylammonium methyl sulfate, which can be referred to as DEMMS. In the above formula (C), when R is a methyl radical, R' is -CH2-CH2-, each R" is a methyl radical, and X
is a CH3S0~ anion, the monomer is the commercially available material (methacryloyloxyethyl)trimethylammonium methylsulfate, sometimes referred to as MTMMS.
Copolymers of acrylamide with said DEMMS monomer are commercially available, for example, an 80:20 acrylamide/DEMMS copolymer. Copolymers of acrylamide with said MTMMS monomer are also commercially available, for ex-ample, Hercules Reten (trademark) 210, a 90:10 acrylamide/MTMMS copolymer;
Hercules Reten (trademark) 220, an 80:20 acrylamide/MTMMS copolymer; Hercules Reten (trademark) 245, a 55~45 acrylamide/MTMMS copolymer; and Hercules Reten (trademark) 260, a 40:60 acrylamide/MTMMS copolymer. The type of co-polymers wherein the number of units from said formula (C) mOnQmer is within the range oE from 10 to 60 mol percent thus comprise another more preferred group of copolymers for use in the practice of the invention. Said copoly-mers can be represented by the~formula ` ~ID73~S

H ~- CH2 C ___ :
o = C o = C : ' _ NH2 _ x '~2 c~30so3Q ,CH2 ~' R" - N - R"

R~
Y ..
(D) :
wherein: R is either hydrogen or a methyl radical; each R" is a methyl radi-cal, or one R" is a methyl radical and the other two R" are each an ethyl radical; and x and y represent the mol percent of said units as set forth above, it being understood that the various copo]ymers do not necessarily consist of alternating units as depicted above in (D). It is also within the scope of the invention or the acrylamide units in the above formula (D) to .
be methacrylamide units, and for a portion of the -NH2 groups in said units to be hydrolyzed.
Thus, it is also within the scope of the invention for the acryl-amide units in the above formula ~D) to be derivatives of either acrylamide or methacrylamide wherein the -NH2 groups can be -NH2 or -OM as defined be-low. Thus, copolymers of said derivatives with the above monomer (C) can be represented by the formula -- 1~ -- ,:

.. .

73~S5 2 ~ ~ --- CHz - C------- ______ O=C ~ O=C

I ~ ' R" - N - R"
y II
(D~) wherein: R, R', R", and X are as defined above in formula (C); R"' is hydro-gen or a methyl radical; in the above Type I monomer units, Z i5 either -N~2 or -OM where~n M is hydrogen, ammonium, or an alkali metal, with said M pre-ferably being hydrogen, sodium, or potassium, and with the proviso that the copolymer contains at least 10 mol percent of said Type I monomer units in which Z is -NH2; x and y are the mol percent values of the respective indi-vidual monomer units I and II, with x being in the range of from 10 to 99, preferably 30 to 95, more preferably 40 to 90, and with y being in the range of from 1 to 90, preferably 5 to 70, more preferably 10 to 60, and with it being understood that the various copolymers do not necessarily consist of alternating monomer units as depicted in formula (D'), e.g., the copolymers are random copolymers as represented by the broken lines connecting said monomer units. It is presently believed that in the above copolymers it is desirable that there be at least 10 mol percent of monomer units containing the -CONH2 group in order for gelation to take place in the presence of an aldehyde gelling agent in accordance with the invention.
Crosslinked polyacrylamides and crosslinked polymethacrylamides, ;
including those at various stages of hydrolysis as described above, and meet-ing the above-stated compatibility requirements, can also be used in the practice of the invention. In general, said crosslinked polyacrylamides can be prepared by the methods described above, ~ut including in the monomeric mixture a suitable a unt of a suitable crosslinking agent. Examples of ;~;

':

"` ~Q736iSS :
crosslinking agents can include methylenebisacrylamide, divinylbenzene, di-vinyl ether, and the like. Said crosslinking agents can be used in small amounts, e.g., up to about 1 percent by weight of the monomeric mixture.
Such crosslinking is to be distinguished from any crosslinking which occurs when solutions of polyrners and the other components of the gelled acidic com-positions of the invention are gelled as described herein.
All the polymers useful in the practice of the invention are char-acterized by high molecular weight. The molecular weight is not critical so long as the polymer has the above-described water-dispersible properties and meets the above-stated compatibility requirements. ~t is preferred that the polymer have a molecular weight of at least 500,000, more preferably at least about 2,0~0,000. The upper limit of molecular weight is unimportant so long as the polymer is water-dispersible, and the gelled acidic composition there-from can be pumped. Thus, it is within the scope of the invention to use polymers having molecular weights as high as 20,000,000 or higher, and meet-ing said conditions.
The amou~t of the above-described polymers used in preparing the gelled acidic compositions of the invention can vary widely depending upon the particular polymer used, the purity of said polymer, and properties de-sired in said compositions. In general, the amount of polymer used will bea water-thickening amount, i.e., at least an amount which will significantly thicken the water to which it is added. For example, amounts in the order of 25 to 100 parts per million by weight (0.0025 to 0.01 weight percent) have been found to significantly thicken water. Dlstilled water containing 25 ppm of a polymer of acrylamide having a molecular weight of about 10 x 106 had a viscosity increase of about 41 percent. At 50 ppm the viscosity in crease was about 106 percent. At 100 ppm the viscosity increase was about 347 percent. As another example, distilled water containing 25 ppm of a polymer of acrylamide having a molecular weight of about 3.5 x 106 had a vis-cosity increase of about 23 percent. At 50 ppm the viscosity increase wasabout 82 percent. At 100 ppm the viscosity increase was about 241 percent.

Generally speaking, amounts of the above-described polymers in the range of from 0.2 to 3, pre~erably from 0.3 to abou~ 2, weight percent, based on the total weight of the composition, can be used in preparing gelled acidic com-positions for use in the practice of the invention.
As a further guide, when the polymer used is one of the above-dis-cussed AMPS or AMPS salt copolymers containing 50 mol percent or more AMPS
or AMPS salt units, the polymer concentration will preferably be in the range of from 0.6 to 3, more preferably 0.75 to about 2 weight percent, based on the total ~eight of the composition. Similarly, when the polymer used is a partially hydrolyzed polyacrylamide or polymethacrylamide, or one of the above-discussed MTMMS or DEMMS copolymers, the polymer concentration will preferably be in the range of from 0.75 to about 2 weight percent~ based on the total weight of the composition. However, it is within the scope of the invention to use amounts outside said ranges. In general, with the proper amounts of acid and aldehyde, the amount of polynler used will determine the consistency of the gel obtained. Small amounts of polymer will usually pro-duce liquid mobile gels which can be readily pumped. Large amounts of poly-mer will usually produce thieker, more viscous, somewhat elastic gels. Gels having a viscosity "too thick to measure" by conventional methods can still be used in the practice of the invention. Thus, -there is really no fixed upper limit on the amount of polymer which can be used so long as the gelled ;
acidic composition can be pumped in accordance with the methods of the inven-tion.
Any suitable water-dispersible aldehyde meeting the above-stated compatibility requirements can be used in the practice of the invention.
Thus, under proper conditions of use, both aliphatic and aromatic monoalde-hydes, and also dialdehydes, can be used. The aliphatic monoaldehydes con-taining from 1 to about 10 carbon atoms per molecule are presently preferred.
Representative examples of such aldehydes include formaldehyde, paraformalde-hyde, acetaldehyde, propionaldehyde, butyraldehydej isobutyraldehyde, valer-aldehyde, heptaldehyde, decanal, and the like. Representative examples of ~73~55 dialdehydes include glyoxal, glutaraldehyde, terephthaldehyde, and the like.
Various mixtures of said aldehydes can also be used in the practice of the invention. The term "water-dispersible" is employed generically herein to include both those aldehydes which are truly water-soluble and those alde-hydes of limited water solubility but which are dispersible in water or other aqueous media to be efEective gelling agents.
Any suitable amount of said aldehydes can be used in the practice of the invention. In all instances the amount of aldehyde used will be a small but effective amount which is sufficient to cause gelation of an aque-ous dispersion of the polymer, the acid, and the aldehyde. As a generalguide, the amount of aldehyde used in preparing the gelled acidic composi-tions of the invention will be in the range of from 0.001 to about 5, prefer-ably 0.004 to about 2, weight percent, based on the total weight of the com-position. Too much aldehyde can be detrimental to gel stability, e.g., cause or promote syneresis and/or cause the gel to become brittle. Those skilled in the art can determine the amount of aldehyde to be used by suitable ex-periments carried out in the light of this disclosure.
Acids useful in the practice of the invention include any acid meet-ing the above-stated compatibility requirements and which is effective in in-creasing the flow of fluids, e.g., hydrocarbons, through the formation andinto the well. Thus, under proper conditions of use, examples of such acids can include inorganic acids such as hydrochloric acid, phosphoric acid, nitric acid, and sulfuric acid; Cl-C4 organic acids such as formic acid, acetic acid, propionic acid, butyric acid, and mixtures thereof, and combinations of in-organic and organic acids. The non-oxidi7ing acids are preferred. The con-centration or strength of the acid can vary depending upon the type of acid, the type of formation being treated, the above-stated compatibility require-ments, and the results desired in the particular treating operation. Gener-ally speaking, the concentration can vary from 0.4 to about 60 weight percent, depending upon the type of acid, with concentrations within tbe range of 10 to 50 weight percent usually preferred, based upon the total weight of the ~073~S5 gelled acidic composition. When an inorganic acid such as hydrochloric acid is used it is presently preferred to use an amount which is sufficient to provide an amount of HCl within the range of from 0.4 to about 35, more pre- -ferably at least about 10, weight percent based on the total weight of the gelled acidic composition. Amounts within the range of about 10 to about 30 weight percent will frequently be practical amounts to use. The acids used in the practice of the invention can contain any of the known corrosion in-hibitors, deemulsifying agents, sequestering agents, surfactants, friction reducers, etc., known in the art, and which meet the above-stated compati-bility requirements.
The gelled ac~dic compositions of the invention are aqueous compo-sitions. They noL~ally contain a significant amount of water. The amount of said water can vary widely depending upon the concentrations of the other components in the compositions, particularly the concentration of the acid.
For example, when an organic acid such as acetic acid is used in a concentra-tion of 60 weight percent, the amount of water present in the composition clearly will be less than when an inorganic acid such as HCl is used in the maximum concentration o abo~t 35 welght percent. Thus, while no precise overall range of water content can be set forth, based on the above-stated overall ranges for the concentrations of said other components, the water content of said co~positions can be in the range of from about 30 to about 99, frequently about 60 to about 90, weight percent. However, amounts of water outside said ranges can be used.
Propping agents can be included in the gelled acidic compositions of the invention if desired. Propping agents which can be used include any of those known in the art, e.g., sand grains, walnut shell fragments, tem-pered glass beads, aluminum pellets, and si~ilar materials, so long as they meet the above-stated compatibility requirements. Generally speaking, it is desirable to use propping agents having particle sizes in the range of 8 to 40 mesh (U.S. Sieve Series). Eowever, particle sizes outside this range can be employed. When propping agents are used they should be made of materials : : ' . , :

~736~5 which are not severely attacked by the acid used during the time they are exposed to said acid.
Any suitable method can be employed for preparing the gelled acidic compositions of the invention. Thus, any suitable m-ixing technique or order of addition of the components of said composition to each other can be em-ployed which will provide a said composition having sufficient stability to degeneration by the heat of the formation (in which the composition is to be used) to permit good penetration of the composltion into, and significant etching of, said formation. However, it is ordinarily preferred to first dissolve or disperse the polymer in water before contacting the polymer with acid. The mixing order can vary with the type of polymer used. Some suit-able~mixing orders, with the components named in order of mixing, include:
water---polymer---acid---aldehyde; water---acid---polymer---aldehyde; acid---polymer---water---aldehyde; and water---polymer---aldehyde---acid; and the like. It is within the scope of the invention to moisten or slurry the poly-mer with à small amount, e.g., about 1 ~o about 6 weight percent based on the weight of the polymer of a small amount of a low molecular weight alcohol, e.g., Cl to C3 alcohols, as a dispersion aid prior to dispersing the polymer in water. It is preferred that there be no undue delay between completing `
the preparation of the gelled acidic composition and its introduction into contact with the formation.
The gelled acidic compositions of the invention can be prepared on the surface ln a suitable tank equipped with suitable mixing means, and then pumped down the well and in~o the formation employing conventional equipment for pumping acidic compositions. ~lowever, it is within the scope of the in-vention to prepare said compositions while they are being pumped down the well. This technique is sometimes referred to as "on the fly". For example, a solution of the polymer in water can be prepared in a tank adjacent the well head. Pumping of this solution through a conduit to the well head can ~hen be started. Then a few feet downstream from the tank a suitable connec-tion can be provided for introducing either the acid or the aldehyde into - 20 ~

said conduit, preferably as an aqueous solution. Then, a few feet farther downstream the other of said acid or aldehyde components can be similarly introduced. As will be understood by those skilled in the art, the rate of introduction of said components into said conduit will depend upon the pump-ing rate of the polymer solution through said conduit. Any of the above-mentioned orders of addition can be employed in said "on the fly" technique.
Mixing orifices can be provided in said conduit, if desired.
It is within the scope of the invention to precede the injection of the gelled acidic composition into the well and out into the formation with a preflush of a suitable cooling fluid, e.g., water. Such ~luids serve to cool the well tubing and formation and extend the useful operating tem-perature range of said compositions. The volume of said cooling fluid so injected can be any suitable volume sufficient to significantly decrease the temperature of the formation being treated, and can vary depending upon the characteristics of the formation. For example, amounts up to 20,000 gallons, or more, can be used to obtain a temperature decrease in the order of 100 to 250F.
The following examples will serve to further illustrate the inven-tion, but should not be considered as unduly limiting on the inven~ion. In carrying out the examples the following general procedure was employed.
A 3.0 weight percent stock solution of polymer or copolymer was prepared at ambient temperature in deionized water. A portion of this stock solution weighed into a beaker was admixed with sufficient water and concen-trated hydrochloric acid (37 weight percent HCl), e.g., to give the desired polymer concentration and acid concentration in individual samples for the test runs. Sufficient aldebyde and water was added to the acid and polymer-containing solution to give about 100 ml of solution. After addition of the aldehyde, the solutions were stirred for about 20 seconds before transferring a 15 ml portion thereof into a Kimax (trademark) No. 500 capillary viscometer for viscosity measurements which are reported herein as the efflux time in seconds corresponding to the time re~uired for the fluid level to drop from .

~73655 one mark to another on the capillary arm of the viscometer. ~fter the firs-t measurement of efflux time, the viscometer containing the sample was placed in a water bath at about 87F. The temperature of the water bath was in-creased at a rate sufficient for the bath temperature to reach about 200F.
in about one hour. During this heating period the efflux time of the sample was repeatedly measured at different temperatures over the entire temperature range. The temperature at which little or no movement of the fluid in the capillary could be detected was designated as the gelation temperature. The onset of gelation was signaled by an increase in the efflux time over a tem-perature interval and this interval is indicated in the examples.
The behavior of the gelled acidic compositions at higher tempera-tures was observed in glass pressure vessels immersed in a heated oil bath at about 250F.
E~AMPLE I
Three concentrations of formaldehyde were used to gel 1 weight percent aqueous solutions of a partially hydrolyzed polyacrylamide contain-ing 28 weight percent hydrochloric acid. The polyacrylamide used was a com-mercially available acryiamide homopolymer (Hercules Reten - trademark -420) having a degree of hydrolysis of 6.3 percent. The results of these runs are summarized below.

3~;S,~;
Table I

Run Gelation Concentration of HCHO
No.Temperature, F.(weight percent) Observations 1No Gelation 0.0 ~o gelation 2 148a 0.004 At 190F. the gel is elastic and clear.
At 199F. the gel bQcame cloudy.

3 108-115b 0.16 Cloudy at 156F., brittle with a little syneresis at ~-168F. Syneresis at 192F. is about 50%.

4 98c 2.0 Rigid gel with a trace of syneresis at 180F. At 200F.
there is at least 50% syneresis.

aGelation began in the temperature interval of 116-144F. as evidenced by an increase in efflux time rom 17 to 23 seconds.

bGelation began in the temperature interval of 100-107F. as evidenced by an increase in efflux time from 15.5 to 18 seconds.
CGelation began in the temperature interval of 77-94F. as evidenced by an increase in efflux time from 19.5 to 35 seconds.
As shown by the results in Table I, the gelation temperature was decreased from about 148F. to 98F. as the formaldehyde concentration was increased from 0.004 weight percent to 2 weight percent formaldehyde.
EXAMPLE II
A gel was prepared from a 28 weight percent aqueous hydrochloric acid solution con~aining 0.24 weight percent acetaldehyde and 1 weight per-cent polyacrylamide (said Hercules Reten ~20). l~is solution gelled at 87F.
in less than one minute, and at 197F. the gel was still clear and elastic.
After 20 minutes at 200F., syneresis was about 10 percent but the gel was still elastic. An unheated portlon of the gel became brittle and exhibited some syneresis after 4.5 hours.
EXAMPLE III
Gelation occurred at 115F. upon heating a 28 weight percent solu~
tion of hydrochloric acid containing 0.4 weight percent isobutyraldehyde and ., ..... ,, ~ ~ ' ,:

~1~73~55 1 weight percent of polyacrylamide (said Reten 420). Gelation began in ~he temperature interval 102-109F. as evidenced hy an increase in efflux time from about 18.6 to 29 seconds. This gel thinned at about 175~F. and broke between 181F. and 187F. giving a homogeneous amber fluid at about 198-200F.
A similar run us-lng 0.4 weight percent n-butyraldehyde gelled at ambient temperature in about one minute. The gel was clear at 160F. and be-gan to thin at 194F. Ater about 30-35 minutes at 200F., syneresis was about 30%.
EXAMPLE IV
A 100 g sample of 28 weight percent aqueous HCl containing 1 weight percent of polyacrylamide (said Reten 420) and 0.5 g valeraldehyda gelled in about one minute at 86F. The gel remained clear and elastic on heating to about 175F. The gel became cloudy at abou~ 185F. and at 200F. syneresis was about 20~, but the remaining gel was elastic.
EXAMPLE V
In a solution containing 28 weight percent HCl, 1 weight percent polyacrylamide (said Reten 420) and 0.6 weight percent heptaldehyde, a gel formed within 2 minutes at 91F. This gel remained elastic up to about 200F.
at which temperature there was approximately 10% syneresis.
EXAMPLE VI
A mixture of 28 weight percent aqueous HCl containing 0.5 weight percent paraformaldehyde and 1 weight percent of a 30:70 acrylamide/sodium-2-acrylamido-2-methylpropanesulfonate copolymer (said Eercules SPX-5025) was heated to 130F. to dissolve the paraformaldehyde. A 15 ml portion of the resulting solution was transferred to a capillary viscometer and on heating to 156F. gelation occurred. Gelation began in the temperature interval of 153-15~F. as evidenced by an incraase in efflux time from 2.7 to 6.5 seconds.
On further heating of this gel to 200F. the gel remained clear and no syn-eresis was evident.
EXAMPLE VII
A solution of 28 weight percent HCl containing 0.35 weight percent glyoxal and 1 weight percent polyacrylamide ~said Reten ~20) gelled at 126F.

10~3~5$

Gelation began in the temperature interval of 116-12~F as evidenced by an increase in efflux time from 17.6 to 32.1 seconds. On heating to 186F a trace of syneresis was evident, but the gel was still elastic. At 200F
syneresis was about 15% and the gel was still elastic.
EXAMPLE VIII
Glutaraldehyde (0.25 weight percent of total reaction mixture) caused gelation of a solution of 28 weight percent aqueous hydrochloric acid containing 1 weight percent polyacrylamide (said Reten 420) in one minute at 89F. On heating to 182F the gel became brittle and syneresis was about 10%.
EXAMPLE IX
A mixture of 28 weight percent HCl containing 0.5 weight percent terephthaldehyde and 1 weight percent polyacrylamide (said Reten 420) gelled at 136-137F. Gelation began in the temperature interval of 124-130F as evi-denced by an increase in efflux time from 25 to 50 seconds. On heating to the temperature range 160-170F, the gel thinned remarkably as evidenced by decreas-ing efflux time.
EXAMPLE X
Several runs were carried out to demonstrate the influence of acid concentration on the gelation of an aqueous solution of 2 weight percent formaldehyde and 1 weight percent polyacrylamide (said Reten 420). The results of these runs which were carried out in a capillary viscometer employing the general procedure described above are summari~ed in Table II.

~0~73~
Table II
Influence of HCl Concentration on Gelation of Polyacylamide (Reten 420) in Aqueous Hydrochloric Acid Acid Concentration Gelation Temperature Comments pH 5.5 No Gelation Approximate acid +
concentration [H30 ]
was 3X10-6 molar pH 1.9 185F The mixture did not 0.4 weight percent HCl gel at room tempera-ture in 2 hours. The mixture gelled in 50 minutes as the bath temperature was in-creased from 74F to 15 ~eight percent HCl 109Fb The mixture gelled in 8-10 minutes as the bath temperature was increased from 85F
to about 105-110F -28 weight percent HCl 98FC The mixture gelled in minutes as the bath temperature was increased from 77F
to 98F
aGelation began in the temperature interval of 177-183F as evidenced by an increase in efflux time from 10.3 to 32.9 seconds.
bGelation began in the temperature interval of 100-103F as evidenced by an increase in efflux time from 30 to 43 seconds.
CGelation began in the temperature interval of 77-94F as evidenced by an increase in efflux time from 19.5 to 35 seconds.
EXAMPLE XI
In order to study the retardation of acid reactivity with formations, e.g., limestone formations, the weight changes of limestone samples contacted with ungelled acid systems, and gelled ac-ld systems in accordance with the invention, were investigated. The limes~one samples were approximately 5/8"
cubes cut from three pieces of core secured from the Smackover limestone in Hopkins County, Texas. The acid systems studied contained: (a) aqueous 28%
HCl alone (Runs 1 and 2); (b) aquecus 28% HCl, and a 30:70 copolymer of acryl-amide and sodium 2-acrylamido-2~methylpropanesulfonate (said Hercules SPX-5025) ~Run 3); gelled system containing aqueous hydrochloric acid, farmaldehyde, and a 30:70 copolymer of acrylamide and sodium 2-acrylamido-2-methylpropane-1-sul-fonate (Runs 4 and 5). The results o~ these runs are summarized in Table III.

1~73~;~5 : `
~ , .
Table III
Retardation of Acid Reactivity With Limestone Wt. Loss of Time of Limestone Run Test Heating Components of State of Sample No. Temperature,P (Min.) Acid System Acid System ~8 1 15~ 2 28% HClUngelled 4.5 2 190-200 2 28~ HClUngelled 4.6 3 200 10 28% HClUngelled 0.43 1% Copolymer 4 150 10 28% HClGelled 0.12 1% Copolym~r 0.16% Formaldehyde 200 10 28% HClGelled 0.2 1% Copolymer 0.16% Formaldehyde Based on the results in Table III (note especially the Weight Loss colu~n) it is concluded that the invention gelled compositions of Runs 4 and 5 retard the rate of reaction between hydrochloric acid and the limestone sample as compared to 28% HCl of control Runs 1 and 2. Control Run 3 demonstrates that the ungelled combination of acid and polymer is less aggressive toward limestone than the acid alone (Runs 1 and 2). However, the ungelled acid/-polymer mixture of Run 3 is more reactive at 200F than the invention gelled composition of Run 5. From these results it is concluded that the inventive gelled acid systems could penetrate more deeply into a subterranean limestone-containlng ~ormation before all the acid was spent.
EXAMPLE XII
Over the temperature range oE 86-196F, a 1 weight percent solution of a homopolymer o~ sodium 2-acrylamido-2-methylpropane-1-sulfonate (Hercules SPX-5185 proprietary designation) did not produce a gel on mixing with 28 weight percent aqueous hydrochloric acid containing 0.4 weight percent formal-dehyde.
EXAMPLE XIII
A solution of 28 weight percent aqueous HCl containing 1 weight per-cent 50:50 acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate copolymer (said Xercules SPX-5023) was gelled with 2 weight percent formaldehyde at about 114F. Gelation began in the temperature interval of 100-107F as evi-~L~736;~

denced by an increase in efflux time from 12.3 to 99 seconds. On gradually heating the gel in a hot water bath to about 200F, the gel never thinned but assumed a brittle consistency. In a similar run using 15 weight percent aqueous hydrochloric acid, the solution gelled at 151F. In another run using 0.5 weight percent of the copolymer in 28 weight percent aqueous hydro-chloric acid, the solution gelled at 159F.
EXAMPLE XIV
A mixture of 28 weight percent aqueous HCl containing 1 weight per-cent 30:70 acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate copolymer (said Hercules SPX-5025) was gelled with 2 weight percent formaldehyde at about 156F. Gelation began in the temperature interval of 144-154F as evidenced by an increase in efflux time from 3.2 to 16 seconds. A duplicate sample was gelled withln 2-3 minutes by immersing the viscometer in a 185F
water bath. The gel was heated at 200F for one hour with little or no vis-ible change.
In a similar run using the same copolymer at a copolymer concentra-tion of 2.0 weight percent, gelation occurred at 123F. Gelation began in the temperature interval of 100-110F as evidenced by an increase in efflux time from 26 to 120 seconds. No gel was produced in a similar run using a copolymer concentration of 0.5 weight percent.
EXAMP~E XV
The gelation of a solution of an acrylamide copolymer (said Hercu-les SPX-5025) using both formaldehyde and acetaldehyde is described in this example. Gelation of a solution containing 1 weight percent 30:70 acrylamide/-sodium 2-acrylamido-2-methylpropanesulfonate copolymer and 28 weight percent hydrochloric acid was effected with 0.13 weight percent formaldehyde and 0.25 weight percent aceta~dehyde at 151F as the bath temperature was increased from 87F to 151F over a period of 2S minutes.
Another portion o~ this gel was transferred to a glass pressure vessel, heated to about 180F in 15 minutes, and then sealed. This sealed vessel was placed in a 250F oil bath and the following observations were made.

~736~S ~ ~
Minutes in 250F sath Observations Gel flows 1/3 into neck of vessel when vessel is tilted; some bubbles are visible No change ~rom above -Slightly darker in color, otherwise no chan~e No change other than darker color Very little change, but will flow into neck if gently shaken An elastic gel; can be shaken about 1/3 into neck of vessel ~0 Gel is stiffer but still elastic 120 A stiff elastic gel The total mixed aldehyde weight used in Example XV amounted to 0.38 weight percent of the total reaction mixture. The high temperature stabllity of the gel prepared in the above Example XV with 30:70 acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate copolymer (said Hercules SPX-5025) was superior to that exhibited by the gels of the following Examples XVI and XVII
formed from solutions containi~g 0.38 weight percent acetaldehyde alone, and 0.4 weight percent formaldehyde alone, respectively. -EXAMPI.E XVI
Gelation of a 28 weight percent aqueous HCl solution containing 1 weight percent of a 30:70 acrylamide/sodium 2-acrylamido-2-methylpropanesul-fonate copolymer (said Hercules SPX-5025) was effected within several minutes with 0.38 weight percent acetaldehyde at ambient temperature.
A 15 ml portion of the test solution was drawn into a capillary viscometer and the viscometer placed in a water bath initially at 118F, and then the bath temperature was increased to about 194F over a period of about 60 minutes. No gelation was observed in the capillary viscometer.
From thls result it is concluded that the gel which formed at ambient tempera-ture in the unhea~ed test solution has less thermal stability than the gel of Example XV in which a mixture of acetaldehyde and formaldehyde was used.
EXAMPLE XVI~
Gelation of a 28 weight percent aqueous ~Cl solution containing 1 weight percent 30:70 acrylamide/sodium 2-acrylamido-2-methylpropanesulfonate , 1~73655 copolymer (said Hercules SPX-5025) was effected with 0.4 weight percent formaldehyde.
A 150 ml test sample of the above composition was heated to 200F
in an uncapped pressure vessel before sealing the bottle and placing it in an oil bath at 250F. The sample gelled during said heating to 200F. The following observations were noted:

Minutes in 250F Bath Observations Large bubbles appeared in the gel The gel is stiff and elastic and will not flow easily into the neck of the bottle when the bottle is tilted.

Gel is thinner and flows readily into the neck of the bottle. Gel is elastic, homogeneous, and very viscous fluid Gel is still homogeneous clear fluid but much less viscous The gel still has some v-lscosity and is still homogeneous Based on the results of Example XVII it is concluded that the gel formed in this 0.4 weight percent formaldehyde run had less thermal stability than did the similarly prepared gel of Example XV which contained a 0.38 weight percent aldehyde mixture of acetaldehyde and formaldehyde.

Based on the results of the examples herein, and particularly Exam-ples XV, XVI, and XVII, it is concluded that gelled acidic compositions of the invention prepared with mixtures of aldehydes, e.g., formaldehyde and acetal-dehyde, are more stable to temperature than such compositions prepared with a single aldehyde. The reason for this surprising result i9 not completely understood at present.
EXAMPLE XVIII
A 64 ml sample of concentrated hydrochloric acid (37 weight percent ~ICl) was mixed with 33 ml of a 3 weight percent solution of a commercial cationic 90:10 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (said Hercules Reten 210), 5 ml of 37 weight percent aqueous formaldehyde, and sufficient water to give a test solution containing 1 weight percent of said cationic copolymer, 28 weight percent HCl, and 2 ~L~73GS5 weight percent formaldehyde. A 15 ml portion of this test solution was drawn into a capillary viscometer and the mixture gelled at 112F. Gelation began in the temperature interval of 101-106F as evidenced by an increase in efflux time from ~.2 to 16 seconds. On further heating to 175F the gel turned cloudy and at 200F syneresis was about 15%. ' ' A duplicate sample was placed in a 200F bath and gelation occurred in one minute. This gel was maintained in the 200F bath for 30 minutes and syneresis was about 50%.
Based on this result, it is concluded that catîonic polyacrylamides, at least those with 10% of the sidechains being cationic, can be used in the practice of the invention.
Additional runs using other cationic polyacrylamides are described in Examples XIX and XX.
EXAMPLB XIX
A 28 weight percent aqueous HCl solution containing 1 weight percent cationic acrylamide copolymer of about 8,000,000 molecular weight and contain-ing cationic functionality provided by the comonomer (acryloyloxyethyl)di-ethylmethylammonium methyl sulfate became gelled in the presence of 0.16 weight percent formaldehyde at 118F in a capillary viscometer. Gelation began in the temperature interval of 107-115F as evidenced by an increase in efflux time from 12.4 to 32 seconds. As the gel was heated syneresis began at about 170F and syneresis was about 50% complete at 200F.
An unheated portion of the test solution gelled within one hour at ambient te,mperature.
EXAMPLF XX
A 28 weight percent aqueous HCl solution containing 1 weight percent of a commercial cationic 80:20 copolymer of acrylamide and (acryloyloxyethyl)-diethylmethylammonium methyl sulfate, having a molecular weight of about 15,000,000, and 0.16 weight percent formaldehyde gelled in a capillary viscom-eter at 115F. Gelation began in the temperature interval of 103-112F as evidenced by an increase in efflux time from 10.8 to 15.2 seconds. On fur-ther heating of this gel to 200F, syneresis became about 50%.

~3~5S

Another sample of the above cationic copolymer test solution was gelled in about 2 minutes at ambient temperature in the presence of 0.24 weight percent acetaldehyde. On further heating of this gel, a viscosity decrease was noted at about 190F and syneresis started at about 200F.
~XAMPLE XXI
A 64 ml sample of concentrated hydrochloric acid (37 weight per-cent HCl) was mixed with 33 ml of a 3 weight percent solution of a commer-cial cationic 80:20 copolymer of acrylamide and (methacryloyloxyethyl)tri-methylammonium methyl sulfate (said Hercules Reten 22~), 0.4 ml of 37 weight percent aqueous formaldehyde, and sufficient water to give a test solu~ion containing 1 weight percent of said cationic copolymer, 28 weight percent HCl, and 0.16 weight percent formaldehyde. A 15 ml portion of the test solution was placed in a capillary viscometer and the mixture gelled at 131F. Gela-tion began in the temperature intervaI of 122-125F as evidenced by an increase in efflux time from 6,4 to 9.2 seeonds. On further heating to 200F
in a total elapsed time of 60 minutes in the hot water bath, syneresis was 10 percent.
FXAMPLE XXII
A 64 ml sample of concentrated hydrochloric acid (37 ~eight percent HCl) was mixed with 33 ml of a 3 weight percent solution of a commercial cationic 55:45 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (said Hercules Reten 245), 0.4 ml of 37 weight percent aqueous formaldehyde, and 2 ml water to give a test solution containing 1 weight percent of said cationic copolymer, 28 weight percent HCl, and 0.16 weight percent formaldehyde. A 15 ml portion oE this test solution was placed in a capillary viscometer and the mixture gelled at 132F. Gelation began in the temperature interval of 124-128F as evidenced by an increase in efflux time from 8.1 to 14.6 seconds. On ~urther heating to 200~F in a total elapsed time of 77 minutes in the hot water bath, there was no syneresis.
After an additional hour at 200F, there was only a trace of syneresis and the gel was still elastic.

~l~t73t~S~

EXAMPLE XXIII
A 96 ml sample of concentrated hydrochloric acid (37 weight percent HCl) was mixed with 50 ml of a 3 weight percent solution of a commercial cationic 55:45 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (said Hercules Reten 245), 2 ml water, 1.5 ml of 37 weight percent aqueous formaldehyde, and 0.5 ml acetaldehyde to give a test solution containing 1 weight percent of said cationic copolymer, 2~ weight percent HCl, 0.4 weight percent formaldehyde, and 0.25 weight percent acetal-dehyde. A llO ml portion of this test solution was placed in a pressure bottle9 heated to 180~F and then placed in a 250F oil bath for a period of 2 hours. The solution gelled during said heating to 180F. The solution remained gelled during 2 hours at 250F but the gel broke up in pieces to a small degree when shaken.
EXAMPLE XXIV
A 64 ml sample of concentrated hydrochloric acid (37 weight per-cent) was mixed with 33 ml of a 3 weight percent solution of a commerclal cationic 40:60 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (said Hercules Reten 260), 0.4 ml of 37 weight percent aqueous formaldehyde, and 2 ml water to give a solution containing 1 weight percent of said cationic copolymer, 28 weight percent HCl, and 0.16 weight percent formaldehyde. A 15 ml portion of this test solution was drawn into a capillary viscometer and the mixture gelled at 131F. Gelation began in the temperature interval of 121-125F as evidenced by an increase in ef1ux time from 11.8 to 60 seconds. After 2 hours at 200F, there was no syneresis.
EXAMPLE XXV
A 64 ml sample of concentrated hydrochloric acid (37 weight percent HCl) was mixed with 33 ml of a 3 weight percent solution of a commercial cationic 40:60 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (said Hercules Reten 260) 0.3 ml acetaldehyde, and 2 ml water to give a solution containing 1 weight percent of said cationic copoly-mer, 28 weight percent ~ICl and 0.235 weight percent acetaldehyde. A 15 ml portion of this test solution was drawn into a capillary viscometer and the `` 1~)73~

mix~ure was a gel at 150F. Gelation began in the temperature il~terval oE 90 to 92F as evidenced by an increase in efflux time from 56 to 197 seconds.
On further heating to 168F, the gel began to thin as the efflux time decreased to 95 seconds. The sample remained a lightly gelled 1uid after 2 hours a~ 200F.
EXANPLE XXVI
A 96 ml sample of concentrated hydrochloric acid (37 weight percent HCl) was mixed with 50 ml of a 3 weight percent solution of a commercial cationic 4~:60 copolymer of acrylamide and (methacryloyloxyethyl)trimethyl-ammonium methyl sulfate (sa~d Hercules Reten 260), 1.5 ml of 37 weight percent aquevus formaldehyde, 0.5 ml acetaldehyde, and 2 ml water to give a test solu-tion containing 1 weight percent of said cationic copolymer, 28 weight percent HCl, 0.4 weight percent formaldehyde, and 0.25 weight percent acetaldehyde. A
110 ml portion of this solution was placed in a pressure bottle, heated to 180F, sealed and placed in a 250F oil bath. During a period of 2 hours at 250F, the gel broke into several lumps on shaking but most of the gel healed by the end of the 2-hour period, A 15 ml sample of the above test solution was placed in a capillary viscometer and the mixture gelled at 110F. Gelation began in the temperature interval of 87-99F as evidenced by an increase in efflux time from 36.5 to 310 seconds.
EXAMPLE XXVII
A 64 ml sample of concentrated hydrochloric acid (37 weight percent HCl) was mixed with 6.7 g of a 15 weight percent solids sample of a commercial cationic homopolymer of (methacryloyloxyethyl)trimethylammonium methyl sulEate (Hercules Hercofloc - trademark - 828 containing Reten - trademark - 300), 5 ml of 37 weight percent aqueous formaldehyde, and 24 ml of water to give a test solution containing 1 weight percent of said cationic homopolymer, 28 weight percent HCl~ and 2 weight percent formaldehyde. A 15 ml portion of this test solution was drawn into a capillary viscometer and the mixture did not gel in a hot water ba~h heated to a temperature of 199~ in a period of 54 minutes. The maximum efflux time during the heating period was 1.7 seconds.

.
.. . : . .. . ;
- . ': ':

-` 10736~

EXAMPLE XXVIII
A 64 ml sample of concentrated hydrochloric acid (37 weight percent HCl) was mixed with 33 ml of a 3 weight percent solution of a commercial par-tially hydroly~ed polyacrylamide (Betz Poly Floc - trademark - 1120, degree of hydrolysis: 25 percent), 0.4 ml of 37 weight percent aqueous formaldehyde9 and sufficient water to give a test solution containing 1 weight percent partially hydrolyzed polyacrylamide, 28 weight percent HCl, and 0.4 weight percent form-aldehyde. A 15 ml portion of this solution was drawn into a capillary vis-cometer and the mix~ure gelled at 113F. Gelation began in the temperature interval of 103-109F as evidenced by an increase in efflux time from 6.9 to 34 seconds. On further heating to 190F syneresis was about 75 percent and increased to about 85 percent at 200F.
EXAMP~E XXIX
A 64 ml sample of concentrated hydrochloric acid (37 weight percent) was mixed with 33 ml of a 3 weight percent solution of a commercial partially hydrolyzed polyacrylamide (Betz Poly Floc - trademark - 1110, degree of hydrolysis 18.5 percent), 0.4 ml of 37 weight percent formaldehyde, and suffi-cient water to give a test solution containing 1 weight percent partially hydrolyzed polyacrylamide, 28 weight percent HCl, and 0.16 weight percent formaldehyde. A 15 ml portion of this solution was drawn into a capillary viscometer and the mixture gelled at 106F. Gelation began in the temperature interval 9~-101F as evidenced by an increase in efflux time from 8.7 to 22.9 seconds. On further heating to 190F syneresis was about 60 percent and lncreased to about 65 percent at 200F.
EXAMPLE XXX
A 159 ml sample of concentrated hydrochloric acid (37 weight per-cent HCl) was mixed with 83 ml of a 3 weight percent solution of a commercial partially hydrolyæed polyacrylamide (Betz Poly Floc - trademark - 1130, degree of hydrolysis: 36.8 percent) and stirred with a spatula. A white gelatinous mass of polymer resulted. This mixture was stirred with a Hamil-ton Beach malt mixer and the white maæs of polymer tended to climb the impel-ler and had to be held down until dispersed. The mix~ure was stirred for .
, 1~73~SS

20-30 seconds at high speed before becoming homogeneous and then stirred at a moderate rate for an additional minute. A 1 ml sample of 37 weight percent aqueous formaldehyde and 7 ml of water were stirred into the homogeneous acid-poly~er mixture to glve a composition containing 1 weight percent par-tially hydrolyzed polyacrylamide, 28 weight percent HCl,, and 0.4 weight percent formaldehyde. A 15 ml portion of this mixture was placed in a capil-lary viscometer and the mixture gelled at 130F. Gelation began in the tem-perature interval 121-124F as evidenced by an increase in efflux time from

5.3 to 18.3 seconds. On further heating to 154F the gel was clear and elastic but at 171F syneresis began and at 173F syneresis was progressing rapidly.
EXAMPL~ XXXI
A sample was prepared by mixing 64 ml water, 33 g of a 3 weight percent solution of a partially hydrolyzed polyacrylamide (said Hercules Reten 420), 0.75 ml concentrated nitric acid (70 weight percent ~03, 1.4 g/ml), 0.6 ml (0.47g) acetaldehyde, and sufficient water to give 100 ml of solution.
A 15 ml portion of this test solution gelled in a capillary viscometer at 175F. Gelation began in the temperature range of 165-172F as evidenced by an increase in the efflux time from 10.1 to 19.3 seconds.
The above run demonstrates that nitric acid can be gelled in the process of the present invention.
Based on the above data, it is concluded that gelled acidic composi-tions comprising an aqueous solution of a suitable polymer of acrylamide having incorporated therein a suitable acid, and a suitable aldehyde, in suitable amounts in accordance with the above-stated compatibility require-ments, are suitable for use in acidizing operations in accordance with the invention. From the viscosity data and the stability da~a of the examples it is further concluded that the components of the gelled compositions have sufficient compatibility with each other to permit good penetration (as defined above) into the formation, and permit malntaining of the compositions in contact with the formation for a period of time usually sufficient for the acid to significantly react with the acid-soluble components of the formation.

1~73655 Thus, it is further concluded that suitable compositions in accordance with the invention could be used advantageously for acidi~ing operations in wells having a depth of up to at least 10,000 feet, and at formation temperatures of up to at least 200F. The use of a preflush cooling fluid injected down the well and into the formation prior to the injection of the gelled acidic composition would extend said ranges of operation. As will be understood by those skilled in the art, the actual attainable ranges of effective acidi~ing operation will depend upon the viscosity of the gelled composition, the formation tempera~ure, the composition of the formation, the rate of injec~
tion of the gelled acidic composition, the acid concentration in said gelled acidic composition, etc.
In those embodiments of the invention wherein a mixture of aldehydes is used it is presently preferred that formaldehyde (or paraformaldehyde) be one of the aldehydes in said mixture because of lower cost and ready avail-ability. It i9 presently preferred that in such mixtures the amount of form-aldehyde (or paraformaldehyde) be within the range of from about 20 to about 75, more preferably about 25 to about 50, weight percent of the mixture of aldehydes.
While certain embodiments of the invention have been desc~ibed for illustrative purposes, the invention is not limited thereto. Various other modifications or embodiments of the invention will be apparent to those skilled in the art in view of this disclosure. Such modifications or embodi-ments arè within the spirit and scope of the disclosure.

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Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for acid treating a porous subterranean formation sus-ceptible of attack by an acid and penetrated by a well bore, which method comprises:
injecting into said formation via said well bore a gelled acidic composition comprising water;
a water-thickening amount of a water-dispersible polymer selected from the group consisting of polyacrylamides and polymethacrylamides; par-tially hydrolyzed polyacrylamides and partially hydrolyzed polymethacryl-amides wherein a portion of the carboxamide groups are initially hydrolyzed to carboxyl groups; crosslinked polyacrylamides and crosslinked polymethacryl-amides; partially hydrolyzed crosslinked polyacrylamides and partially hydro-lyzed crosslinked polymethacrylamides wherein a portion of the carboxamide groups are initially hydrolyzed to carboxyl groups; copolymers of acrylamide or methacrylamide with another ethylenically unsaturated monomer copolymer-izable therewith, sufficient acrylamide or methacrylamide being present in the monomer mixture to impart said water-dispersible properties to the result-ing copolymer when it is mixed with water; and mixtures thereof;
an amount of an acid which is capable of, and sufficient for, reacting with a significant amount of the acid-soluble components of said formation;
a small but effective amount of a mixture of at least two water-dispersible aldehydes which is sufficient to cause gelation of an aqueous dispersion of said polymer, said acid, and said aldehydes;
said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form a said composition having sufficient stability to degeneration by the heat of said formation to permit good pene-tration of said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.

2. A method according to claim 1 wherein:
the amount of said polymer is within the range of from 0.2 to about 3 weight percent based upon the total weight of said composition;
the amount of said aldehydes is within the range of from 0.001 to 5 weight percent, based upon the total weight of said composition; and the amount of said acid is within the range of from 0.4 to 60 weight percent, based on the total weight of said composition.

3. A method according to claim 2 wherein:
said aldehydes are aliphatic monoaldehydes containing up to 10 carbon atoms per molecule; and said acid is hydrochloric acid, and the amount thereof is sufficient to provide an amount of HCl within the range of from 0.4 to about 35 weight percent.

4. A method for acid treating a porous subterranean formation susceptible of attack by an acid and penetrated by a well bore, which method comprises:
injecting into said formation via said well bore a gelled acidic composition comprising water;
a water-thickening amount of a water-dispersible polymer selected from the group consisting of polyacrylamides and polymethacrylamides wherein up to about 45 percent of the carboxamide groups can be initially hydrolyzed to carboxyl groups;
an amount of an acid which is capable of, and sufficient for react-ing with a significant amount of the acid-soluble components of said formation;
a small but effective amount of a mixture of at least two water-dispersible aldehydes which is sufficient to cause gelation of an aqueous dis-persion of said polymer, said acid, and said aldehydes;
said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form a said composition having sufficient stability to degeneration by the heat of said formation to permit good penetration of said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimu-late the production of fluids therefrom.

5. A method according to claim 4 wherein:
the amount of said polymer is within the range of from 0.2 to about 3 weight percent, based on the total weight of said composition;
the amount of said aldehydes is within the range of from 0.001 to 5 weight percent, based on the total weight of said composition; and the amount of said acid is within the range of from 0.4 to 60 weight percent, based on the total weight of said composition.

6. A method according to claim S wherein: said acid is hydro-chloric acid, and the amount thereof is sufficient to provide an amount of HCl within the range of from 0.4 to about 35 weight percent, based on the total weight of said composition; and said aldehydes are aliphatic monoalde-hydes containing up to 10 carbon atoms per molecule, and the amount thereof is within the range of from 0.004 to about 2 weight percent, based on the total weight of said composition.

7. A method according to claim 4 wherein:
said polymer is a partially hydrolyzed polyacrylamide wherein not more than about 20 percent of the carboxamide groups are initially hydrolyzed to carboxyl groups, and the amount thereof is within the range of from about 0.75 to about 2 weight percent, based on the total weight of said composition, said acid is hydrochloric acid, and the amount thereof is suffi-cient to provide an amount of HCl within the range of about 10 to about 30 weight percent, based on the total weight of said composition; and said aldehydes are selected from the group consisting of the C1 to C10 aliphatic monoaldehydes, and the amount of said aldehydes is within the range of from 0.004 to about 2 weight percent, based on the total weight of said composition.

8. A method according to claim 7 wherein said mixture of aldehydes comprises formaldehyde and acetaldehyde.

9. A method for acid treating a porous subterranean formation susceptible of attack by an acid and penetrated by a well bore, which method comprises:
injecting into said formation via said well bore a gelled acidic composition comprising water;
a water-thickening amount of a water-dispersible copolymer of acrylamide or methacrylamide with another ethylenically unsaturated monomer copolymerizable therewith, sufficient acrylamide or methacrylamide being present in the monomer mixture to impart said water-dispersible properties to the resulting copolymer when it is mixed with water;
an amount of an acid which is capable of, and sufficient for, reacting with a significant amount of the acid-soluble components of said formation;
a small but effective amount of a mixture of at least two water-dispersible aldehydes which is sufficient to cause gelation of an aqueous dispersion of said polymer, said acid, and said aldehydes;
said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form a said composition having sufficient stability to degeneration by the heat of said formation to permit good penetration of said composition into said formation; and maintaining said composition in said formation in contact therewith for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.

10. A method according to claim 9 wherein:
the amount of said copolymer is within the range of from 0.2 to about 3 weight percent, based on the total weight of said composition;

the amount of said aldehydes is within the range of from 0.001 to 5 weight percent, based on the total weight of said composition and the amount of said acid is within the range of from 0.4 to 60 weight percent, based on the total weight of said composition.

11. A method according to claim 10 wherein:
said aldehydes are aliphatic monoaldehydes containing up to 10 carbon atoms per molecule; and said acid is hydrochloric acid, and the amount thereof is suffi-cient to provide an amount of HCl within the range of from 0.4 to about 35 weight percent.

12. A method according to claim 11 wherein:
said copolymer is a random copolymer represented by the formula wherein: R and R"' are each either hydrogen or a methyl radical; R' is an alkylene radical containing from 1 to 24 carbon atoms or is an arylene radi-cal containing from 6 to 10 carbon atoms; each R" is an alkyl radical contain-ing from 1 to 6 carbon atoms; X is any suitable anion; in the above Type I
monomer units, Z is either -NH2 or -OM wherein M is hydrogen, ammonium, or an alkali metal, with the proviso that the copolymer contains at least 10 mol percent of said Type I monomer units in which Z is -NH2; and x and y are the mol percent values of the respective individual monomer units I and II, with x being in the range of from 10 to 99, and with y being in the range of from 1 to 90.

13. A method according to claim 12 wherein:
the amount of said copolymer is within the range of from 0.75 to 2 weight percent, based on the total weight of said composition;
said acid is hydrochloric acid and the amount thereof is sufficient to provide an amount of HCl within the range of about 10 to about 35 weight percent, based on the total weight of said composition; and said aldehyde is a mixture of at least two aldehydes selected from the group consisting of the C1 to C10 aliphatic monoaldehydes, and the amount of said aldehydes is within the range of from 0.004 to 2 weight percent, based on the total weight of said composition.

14. A method according to claim 13 wherein said mixture of alde-hydes comprises formaldehyde and acetaldehyde.

15. A method according to claim 14 wherein, in said formula:
R and R"' are each hydrogen;
R' is a -CH2 - CH2 - radical;
one R" is a methyl radical and the other two R" are each an ethyl radical; and X is a methylsulfate anion.

16. A method according to claim 14 wherein, in said formula:
R is a methyl radical;
R' is a -CH2 - CH2 - radical;
each R" is a methyl radical;
R"' is hydrogen; and X is a methylsulfate anion.

17. A gelled acidic composition, suitable for matrix acidizing or fracture-acidizing of a porous subterranean formation susceptible of attack by an acid, comprising:
water;
a water-thickening amount of a water-dispersible polymer selected from the group consisting of polyacrylamides and polymethacrylamides wherein up to about 45 percent of the carboxamide groups can be initially hydrolyzed to carboxyl groups; crosslinked polyacrylamides and crosslinked polymetha-crylamides wherein up to about 45 percent of the carboxamide groups can be initially hydrolyzed to carboxyl groups; copolymers of acrylamide or meth-acrylamide with another ethylenically unsaturated monomer copolymerizable therewith, sufficient acrylamide or methacrylamide being present in the mono-mer mixture to impart said water-dispersible properties to the resulting co-polymer when it is mixed with water;
an amount of a non-oxidizing acid which is capable of, and suf-ficient for reacting with a significant amount of the acid-soluble components of said formation; and a small but effective amount of a mixture of at least two water-dispersible aldehydes which is sufficient to cause gelation of an aqueous dispersion of said polymer, said acid, and said aldehydes;
said polymer, said acid, and said aldehydes, in the amounts used, being sufficiently compatible with each other in an aqueous dispersion thereof to permit said gelation and thus form a said composition having sufficient stability to degeneration by the heat of said formation to permit good pene-tration of said composition into said formation when injected thereinto and the maintenance of said composition in said formation in contact therewith for a period of time sufficient for the acid in said composition to react significantly with the acid-soluble components of said formation and stimulate the production of fluids therefrom.

18. A composition according to claim 17 wherein:
the amount of said polymer is within the range of from 0.2 to about 3 weight percent based upon the total weight of said composition;
the amount of said aldehydes is within the range of from 0.001 to 5 weight percent based upon the total weight of said composition; and the amount of said acid is within the range of from 0.4 to 60 weight percent, based on the total weight of said composition.

19. A composition according to claim 18 wherein:
said polymer is a partially hydrolyzed polyacrylamide wherein not more than about 45 percent of the carboxamide groups are initially hydrolyzed to carboxyl groups, and the amount thereof is within the range of from 0.1 to about 3 weight percent, based on the total weight of said composition;
said acid is hydrochloric acid, and the amount thereof is sufficient to provide an amount of HCl within the range of from 0.4 to about 35 weight percent, based on the total weight of said composition; and said aldehydes are selected from the group consisting of the C1 to C10 aliphatic monoaldehydes, and the amount of said aldehydes is within the range of from 0.004 to about 2 weight percent, based on the total weight of said composition.

20. A composition according to claim 19 wherein said mixture of aldehydes comprises formaldehyde and acetaldehyde.

21. A composition according to claim 18 wherein:
said polymer is a random copolymer represented by the formula wherein: R is hydrogen or a methyl radical; each R" is a methyl radical, or one R" is a methyl radical and the other two R" are each an ethyl radical;
R"' is hydrogen or a methyl radical; in the above Type I monomer units, Z is either -NH2 or -OM wherein M is hydrogen, sodium, or potassium, with the proviso that the copolymer contains at least 10 mol percent of said Type I
monomer units in which Z is -NH2; and x and y are the mol percent values of the respective monomer units I and II, with x being in the range of from 10 to 99, and with y being in the range of from 1 to 90;
the amount of said copolymer is within the range of from 0.75 to about 2 weight percent, based on the total weight of said composition;
said acid is hydrochloric acid and the amount thereof is sufficient to provide an amount of HCl within the range of 0.4 to about 35 weight per-cent, based on the total weight of said composition; and said aldehydes are selected from the group consisting of the C1 to C10 aliphatic monoaldehydes, and the amount of said aldehydes is within the range of from 0.004 to about 2 weight percent, based on the total weight of said composition.

22. A composition according to claim 21 wherein said mixture of aldehydes comprises formaldehyde and acet-